The strategy is NOT to have gas and petrol injection times the same. After all, they are different injectors using different fuel - never going to be the same.

The approach is to get petrol time - while running on gas - to be the same as petrol time - while running on petrol - at the same load conditions.This usually involves switching between fuels, if you can only see times in mS.The job is made easier if you can look at fuelling trim values from an OBD reader. Getting the trim values the same is the target.

WowI thought I was late...on the forum.. thanks for the reply. Im a little confused whe you say petrol time to be the same on gas as petrol, is that not the same as injector time. Just a bit confused.....

I think im running lean cuase the car seems to be struggeling a bit on power, I know there are otqher reasonsFor that also when driving around town idle seems bad. But I must admit that I wqs having some trouble with air filters...a long story.... but thats pretty much been sorted now and I think that has made a difference. Also since the last the RC was change I lost about 20 mpg per tank. And the other thing taht makes me think I may he lean is the comment about trip when it mentionz that the trip computer needs to show the same for running on gas and mpg, based on that ia run lower mpg than petrol, which I think implies I, m lean. But I, not too sure is that vailc for my mercedes CL 420 as I don't really have a trip computer rather a mpg gauge

Im a little confused whe you say petrol time to be the same on gas as petrol, is that not the same as injector time.

You have two sets of injectors, one per cylinder for petrol and a second one per cylinder for gas. The gas ECU doesn't replace the petrol ECU but piggybacks onto it, the car doesn't realise it isn't running on petrol. The petrol ECU sends a pulse of a predetermined duration to fire the petrol injector but this pulse is intercepted by the gas ECU which adds a fiddle factor to the duration and fires the gas injectors. So if at any given revs and load the petrol injector needs to be open for say, 5mS, at that same revs and load the gas injector needs to be open for 6mS, then it is the job of the gas ECU to add the additional 1mS. If the gas ECU has been programmed correctly, the lambda sensor reports to the petrol ECU that the mixture is correct and everything is happy. If however, the gas ECU is only adding 0.5mS then the lambda sensor reports that the mixture is lean so the petrol ECU will increase the petrol injector pulse to 5.5mS to get the mixture correct. For this reason you need to be looking at the petrol injector times when on petrol and gas to ensure that they are the same and the petrol ECU isn't having to make any corrections. What the gas injector times are is pretty much irrelevant. If they are much higher than the petrol times (meaning the gas ECU is adding a huge fiddle factor) then that is down to gas pressure or nozzle size rather than programming.

Sounds to me like your installer is fiddling in the dark rather than actually looking at the data and working out what the problem is. If the car is indeed running lean then the programming is so far out that the petrol ECU cannot adjust the pulse duration enough to get the mixture correct.

jookul wrote:

I don't really have a trip computer rather a mpg gauge

Same thing. You have two factors that affect how much fuel the engine is using. Fuel pressure, which is fixed and injector pulse duration. The mpg meter knows what the petrol pressure is so by looking at the injector pulse duration can work out how much petrol is being used and display an mpg figure. If the gas ECU programming means the fiddle factor is wrong then the petrol ECU will adjust the pulse duration which will make your mpg meter show a different mpg figure between fuels.

Hi guysThanks for the explaination. That makes much more sence. I had a word with my guy yesterday and I think he is in the ball park. Though I will mention your explaination as I dont think he is looking at it from the above prospective. We are also trying to get the STFT and the LTFT but that's not as easy as it sounds on a 98 CL 420. I mentioned that I had a air filter issue that is resolved now and has improved the situation. So there are a number of things playing out here but I hope we are getting through them...I'll. Be back with my guy during the week to see how we go again

just a quick qiestion. It looks like we might be able to get the short and long term fuel trim values, if we can the question is this.... when you are setting up the gas for the fuel trim values must you have the car under load ie taking a live reading while driving the car and then making changes on the gas system to ensure the trim values are the same.

My installer says the car must be driving while u are setting up, but I read sometinh about the Prins VSI system that makes me think that all you have to do is read the trim values and just match up the gas ie the car does not need to be driving under load.

I just want to understand which is the correct way to do this..... thnaks again for all of the support

You are setting up a map that will have the revs the engine is doing and will then need a setting for all eventualities at all revs. So as well as setting it at idle, you'll need to set it at mid revs on the flat, going uphill at a constant speed, accelerating uphill, etc. OK, so you don't have to do each eventuality at every point in the rev range, but you need to do the extremes so you can fill in the gaps.

So the latest.... I took it back to in installer and he checked out the tuning and he says its quite close or at least he is happy to how close it is... ie petrol and gas timings are very close as you guys sdescribe before. I mentioned that I wNwanted to make sure that the car was not running too lean. Based on thT and the drive he has now changed the RC to 102 so I suppose I need to wait and see.... but if u follow tne fuel gauge analology mentioned above then I still think its a little lean.

Also a quick question on the Prins VSI my installer says that if a lambda alaram of "too rich too long" comes up. He says that it means that there is too much oxegen being sensed by the lamba and not too much fuel (as I originally thought). I can see the logic in the statment but is that correct. Bear in mind im referencing the Prins Vsi sytem only

Prins fault report "too xxx too long" means that the Prins system has not seen the sensor output change, within some defined time.The time is defined by the parameter "ETL".When was the last time you saw that error?

Some Mercs have 0.8 to 1.6 Lambdas - could cause too rich too long errors if the ECU is expecting 0 to 1v.

Interesting how the ECU knows the driver hasn't got his foot to the floor and the rich mixture isn't supposed to be maintained for a long time if no TPS connection is made. Sure, it could determine this from injector pulse duration, but if the system was fitted to a low revving big block V8 that does 25ms+ pulse duration then the system wouldn't be effective... The same is true for too lean too long errors - How does the system know that the driver isn't going downhill from the top of a mountain and the mixture should stay lean for maybe an hour?

I think Jookul had said that the error being logged was due to over pressure.. In which case the reducer pressure could be lowered or the maximum pressure raised in the calibration. The situation might be that the pressure is OK during normal driving but on over-run with injectors still being pulsed but only for very short duration, the pressure rises above the maximum pressure causing this error and the system will switch back to petrol with an error logged. I'd be looking at the pressure settings and readings before going much further...

thanks for all the replies I think are getting better, as mentioned i haven't seen the error since the ETL was changed to 58 sec and the current settings is the evaporator is set to 2.09 PSI and the RC is 100. I still not too sure if the RC setting is too lean

I can't get the fuel trim values for the cart as it doesn't have an ODB connection... My gas installer thinks the setting are fairly good,but if you follow the comments above about using the MPG Computer to determine whether u are set up correctly or not then I think I am still a bit lean.

Previously in the thread one of the guys said he has set up a CL 500 to and RC of 110, so i may go to that for a week to see what happened but I'm not too sure yet

My MPG on this car is 18 MPG and that is driving very carefully I was hoping to get a bit more (The 18MPG includes the petrol usage) but I'm slowly coming to terms that this is as good as it gets. I was hoping for 20 MPG. On petrol I was getting around 23MPG

thanks for all the replies I think are getting better, as mentioned i haven't seen the error since the ETL was changed to 58 sec and the current settings is the evaporator is set to 2.09 PSI and the RC is 100. I still not too sure if the RC setting is too lean

(deleted bit not relevant to my reply below)

My MPG on this car is 18 MPG and that is driving very carefully I was hoping to get a bit more (The 18MPG includes the petrol usage) but I'm slowly coming to terms that this is as good as it gets. I was hoping for 20 MPG. On petrol I was getting around 23MPG

Does anybody now any tricks or setup that may help

If the problem was the ETL then you'll still have problem if going downhill for long enough... it will now trip back to petrol after 58 seconds instead of whatever it was before (like I said in previous post).

I like to see RC around 105 (not accounting for any negative offset) on typical engines with 6500red line and 16ms full load, that's a bit of a multiplier to help low load smoothness but not too much to cause injector full open issues at full load. If you have similar petrol inj figures when you switch between fuels I'd leave it as it is though rather than getting too involved in that.

If the Lambda is flicking properly during closed loop mode then you won't be able to increase actual mpg during closed loop mode running and I doubt there is anything you can do. Of course, don't take the mpg gauge in the vehicle as being correct, you'll need to calculate mpg doing the fill and run until empty method and over several fill-ups.

There may be slight gains to be made during open loop mode (high manifold pressures i.e. high engine loads) if the mixture is too lean or rich at the top end. Use a voltmeter on the Lambda(s)? I believe some Prins systems are capable of being set up to keep lambda close to at an actual voltage during open loop mode (i.e. working as a slave during petrol closed loop running but applying their own closed loop function to keep lambda at a certain level during open loop mode).. No good if the ECU is expecting a 0to1V signal and Lambdas are 0.8 to 1.6v, or high load open loop running will be leaned off by the ECU. Doubt it though.

I have not got the RC changed to 105 with an offset of 7. Im going to run wuth that for afew weeks and see ehat happens

Check the top end fuelling....

Not only do you need to multiplier and offset, but also working pressure....

If you started with zero offset and a higher multiplier, but now have a 7 offset and lower multiplier, the mixture at the bottom end may be the same but the mixture at the top end leaner! So for example vehicles that run correct at idle but get leaner at the top, you'd need to increase the working pressure with the same multiplier, and/or increase the multiplier, and/or have a negative offset...

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